The invention relates to a method of making pelletized fuel containing plastic and cellulose particles in which a feed stream containing plastic and cellulose particles is supplied to a pelletizing device.
The method is used in particular in processes for recycling household and industrial waste streams into pelletized fuel. Cellulose particles originate from for example paper, wood, nappies, bandages and textile. Plastic particles originate from for example packaging materials, in particular polyethylene film. Such pelletized fuel recyclate has a high heat of combustion and is used as fuel in furnaces. The invention also relates to pelletized fuel, to the use of the pelletized fuel as secondary fuel and to different methods of firing furnaces with the pelletized fuel.
The commercially available pelletized fuel recyclate has hitherto been used virtually exclusively under non-critical combustion conditions, as in fluid-bed furnaces. Because of the long residence times in such incineration furnaces, fewer requirements are imposed with respect to the properties of the pelletized fuel and the methods of making it. The known pellets are virtually not used as secondary fuel under more critical combustion conditions, as for example in blast-furnaces and power-station furnaces fired with pulverized coal. As a result, the potential demand for pelletized fuel recyclate lags far behind the tremendous potential supply. This is very disadvantageous because a very large amount of polluting waste must be dumped or processed at high costs while a huge source of energy remains untapped.
U.S. Pat. No. 5,342,418 describes a method of making pelletized fuel from household waste, in particular from nappies and sanitary napkins, for use as secondary fuel in a mixture of coal for firing furnaces. The plastic content of the pellets is to this end chosen to lie between 10 and 40 wt. %. The properties of the pelletized fuel, such as the dimensions, the composition and the density, have been chosen in the light of its use as secondary fuel in a mixture containing the coal as the primary fuel. These pellets are produced by using a shredder and a hammer mill to reduce the particle size in a waste stream consisting of plastic and cellulose particles and supplying the feed stream obtained to a pelletizing device. It is described that the pellets obtained are lightly bound and break easily and that harder pellets can be obtained by reducing the plastic content and increasing the moisture content. According to U.S. Pat. No. 5,342,418, a higher plastic content implies an even lower hardness, as a result of which the pellets will break even more easily. It is also mentioned that higher plastic contents of up to 60 wt. % are actually possible, but that such pelletized fuel is not suitable for use as secondary fuel for coal and has to be burned in special pellet incinerators. The plastic content and the cellulose content are here and hereinafter each time expressed in percentages of the total dry weight of the plastic and the cellulose. Where here and hereinafter reference is made to properties of the fuel pellet or of particles in a collection of particles, this is intended to imply the average value of a sufficiently large number of pelletized fuel or particles. xe2x80x9cFeed streamxe2x80x9d is here and hereinafter each time understood to be the stream that is fed to the pelletizing device.